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Chapter 1 - Plate Tectonics

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Chapter 1 
Plate tectonics 
A perspective 
Plate tectonics, which has so profoundly influenced geo-
logic thinking since the early 1970s, provides valuable 
insight into the mechanisms by which the Earth's crust 
and mantle have evolved. Plate tectonics is a unifying 
model that attempts to explain the origin of patterns of 
deformation in the crust, earthquake distribution, contin-
ental drift, and mid-ocean ridges, as well as providing a 
mechanism for the Earth to cool. Two major premises of 
plate tectonics are: 
1 the outermost layer of the Earth, known as the litho-
sphere, behaves as a strong, rigid substance resting 
on a weaker region in the mantle known as the 
2 the lithosphere is broken into numerous segments or 
plates that are in motion with respect to one another 
and are continually changing in shape and size (Fig-
ure 1.1/Plate 1). 
The parental theory of plate tectonics, seafloor spread-
ing, states that new lithosphere is formed at ocean ridges 
and moves away from ridge axes with a motion like that 
of a conveyor belt as new lithosphere fills in the result-
ing crack or rift. The mosaic of plates, which range from 
50 to over 200 km thick, are bounded by ocean ridges, 
subduction zones (in part coUisional boundaries), and 
transform faults (boundaries along which plates slide 
past each other) (Figure 1.1/Plate 1, cross-sections). To 
accommodate the newly-created lithosphere, oceanic 
plates return to the mantle at subduction zones such that 
the surface area of the Earth remains constant. Harry 
Hess is credited with proposing the theory of seafloor 
spreading in a now classic paper finally published in 
1962, although the name was earlier suggested by Robert 
Dietz in 1961. The basic idea of plate tectonics was 
proposed by Jason Morgan in 1968. 
Many scientists consider the widespread acceptance 
of the plate tectonic model as a 'revolution' in the Earth 
Sciences. As pointed out by J. Tuzo Wilson in 1968, 
scientific disciplines tend to evolve from a stage prim-
arily of data gathering, characterized by transient hypo-
theses, to a stage where a new unifying theory or theories 
are proposed that explain a great deal of the accumu-
lated data. Physics and chemistry underwent such revo-
lutions around the beginning of the twentieth century, 
whereas the Earth Sciences entered such a revolution in 
the late 1960s. As with scientific revolutions in other 
fields, new ideas and interpretations do not invalidate 
earlier observations. On the contrary, the theories of 
seafloor spreading and plate tectonics offer for the first 
time unified explanations for what, before, had seemed 
unrelated observations in the fields of geology, paleon-
tology, geochemistry, and geophysics. 
The origin and evolution of the Earth's crust is a tan-
talizing question that has stimulated much speculation 
and debate dating from the early part of the nineteenth 
century. Some of the first problems recognized, such 
as how and when did the oceanic and continental crust 
form, remain a matter of considerable controversy even 
today. Results from the Moon and other planets indicate 
that the Earth's crust may be a unique feature in the 
Solar System. The rapid accumulation of data in the fields 
of geophysics, geochemistry, and geology since 1950 
has added much to our understanding of the physical 
and chemical nature of the Earth's crust and of the pro-
cesses by which it evolved. Evidence favours a source for 
the materials composing the crust from within the Earth. 
Partial melting of the Earth's mantle produced magmas 
that moved to the surface and formed the crust. The 
continental crust, being less dense than the underlying 
mantle, has risen isostatically above sea level and hence 
is subjected to weathering and erosion. Eroded materials 
are partly deposited on continental margins, and partly 
returned to the mantle by subduction to be recycled and 
perhaps again become part of the crust at a later time. 
Specific processes by which the crust formed and evolved 
are not well-known, but boundary conditions for crustal 
processes are constrained by an ever-increasing data base. 
In this book, physical and chemical properties of the 
2 Plate Tectonics and Crustal Evolution 
Figure 1.1 Map of the major lithospheric plates on Earth. Arrows are directions of plate motion. Filled barbs, convergent 
plate boundaries (subduction zones and coUisional orogens); single lines, divergent plate boundaries (ocean ridges) and 
transform faults. Cross-sections show details of typical plate boundaries. Artwork by Dennis Tasa, courtesy of Tasa Graphic 
Arts, Inc. 
Plate tectonics 3 
Plate Tectonics and Crustal Evolution 
' O 
Q . 
1000 2000 3000 4000 
Depth (km) 
— -H2000 
Figure 1.2 Distribution of 
average compressional (Vp) and 
shear wave (Vs) velocities and 
average calculated density (p) in 
the Earth. Also shown are 
temperature distributions for 
whole-mantle convection (TW) and 
layered-mantle convection (TL). 
Earth are described, and crustal origin and evolution are 
discussed in the light of mantle dynamics and plate tec-
tonics. Included also is a discussion of the origin of the 
atmosphere, oceans, and life, which are all important 
facets of Earth history. Finally, the uniqueness of the 
Earth is contrasted with the other planets. 
Structure of the Earth 
First of all we need to review what is known about the 
structure of planet Earth. The internal structural of the 
Earth is revealed primarily by compressional (P-wave) 
and shear (S-wave) waves that pass through the Earth in 
response to earthquakes. Seismic-wave velocities vary 
with pressure (depth), temperature, mineralogy, chemi-
cal composition, and degree of partial melting. Although 
the overall features of seismic-wave velocity distribu-
tions have been known for some time, refinement of 
data has been possible in the last ten years. Seismic-
wave velocities and density increase rapidly in the re-
gion between 200 and 700 km deep. Three first-order 
seismic discontinuities divide the Earth into crust, 
mantle and core (Figure 1.2): the Mohorovicic discon-
tinuity, or Moho, defining the base of the crust; the 
core-mantle interface at 2900 km; and, at about 5200 
km, the inner-core/outer-core interface. The core com-
prises about sixteen per cent of the Earth by volume and 
thirty-two per cent by mass. These discontinuities reflect 
changes in composition or phase, or both. Smaller, but 
very important velocity changes at 50-200 km, 410 km, 
and 660 km provide a basis for further subdivision of 
the mantle, as discussed in Chapter 4. 
The major regions of the Earth can be summarized as 
follows with reference to Figure 1.2: 
1 The crust consists of the region above the Moho, 
and ranges in thickness from about 3 km at some 
oceanic ridges to about 70 km in coUisional orogens. 
2 The iithosphere (50-300 km thick) is the strong 
outer layer of the Earth, including the crust, that 
reacts to many stresses as a brittle sohd. The astheno-
sphere, extending from the base of the Iithosphere 
to the 660-km discontinuity, is by comparison a weak 
layer that readily deforms by creep. A region of low 
seismic-wave velocities and high attenuation of 
seismic-wave energy, the low-velocity zone (LVZ), 
occurs at the top of the asthenosphere and is from 
50-100 km thick. Significant lateral variations in 
density and in seismic-wave velocities are common 
at depths of less than 400 km. 
The upper mantle extends from the Moho to the 
660-km discontinuity, and includes the lower part of 
the Iithosphere and the upper part of the astheno-
sphere. The region from the 4l0-km to the 660-km 
discontinuity is known as the transition zone. These 
two discontinuities, as further

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